1. Field of the Invention
The present invention relates to the manufacture of cylindrical electromagnets composed of a plurality of annular coils. The present invention finds particular relevance with respect to the manufacture of superconducting coils for magnetic resonance imaging (MRI) systems, but may be applied to other types of electromagnet, both superconducting and resistive.
2. Description of the Prior Art
Several methods for manufacturing cylindrical electromagnets have been used in the past. Conventionally, a cylindrical former is produced, for example of aluminum stainless steel or GRP, into which annular cavities of rectangular cross-section are formed. Coils of wire, for example superconducting wire, are then wound into these annular cavities. The resulting assembly may be impregnated with a thermosetting resin to retain the wire within the coils in position.
More recently, methods have been proposed in which coils are wound into cavities within a mold, with a supporting structure placed on the radially outer surface of the coils, and the resulting structure impregnated with thermosetting resin. The mold is then removed leaving the impregnated coils bonded to the supporting structure.
Co-pending United Kingdom patent application No. GB0912367.0 describes a solenoidal magnet arrangement in which the coils are bonded by their radially outer surfaces to a radially outer mechanical support structure.
United States Patent Application Publication No. 2007/0247263 describes another solenoidal magnet arrangement in which the coils are bonded by their radially outer surfaces to a radially outer mechanical support structure, and similar arrangements in which the supporting structure is placed in the mold first, with the coils being wound over the supporting structure.
A drawback of these methods is that the relative positions of the coils cannot be adjusted. While it may be theoretically possible to remedy deficiencies in the resultant magnetic field by adjustment of the relative positioning of certain coils, in practice this is not possible with coils structures which have been bonded to a support structure by resin impregnation.
Similarly, it has been found impractical to remove a single coil from such an assembly for repair or replacement, leading to scrapping of complete cylindrical magnet coil assemblies, even where only a single coil is defective.
Any new method of assembling annular coils must ensure that the coils are accurately positioned relative to one another, and that the coils will not move under the significant forces which they are subjected to in use.
Support structures for coils of superconducting solenoid magnets are described in U.S. Pat. Nos. 4,896,128 and 4,467,303. In each case, a coil is clamped onto a partial outer former by compressing the outer radial extremity of the coil between a step formation formed on the radially inner surface of the partial former and a clamp piece or clamp ring. In use, axial forces of several tonnes will act on the coils. As the coils are restrained only at their radially outer extremity, by compression at their axially outer extremities, severe stresses will build up within the coils. At least in U.S. Pat. No. 4,467,303, the coils are restrained by thermal shrinking of the outer support structure (e.g. column 4 lines 7-18). The resultant hoop, radial and axial stresses may cause damage to the structure of the coils, by cracking the impregnating material. Resulting movement of turns of the coils may result in a quench. The coils will be subjected to severe mechanical loading at the points of contact of the coil with the step formation and the clamp pieces or clamp ring. The present invention provides superconducting solenoidal magnet arrangements in which mechanical loading of the coils is not concentrated at localized points of contact. Rather, according to the present invention, each coil is bonded over its radially outer surface to an outer mechanical support structure. This avoids any local points of high mechanical stress, for example high shear stress within the coil structure such as caused by conventional coil supporting formations.
The mechanical load resulting from the axial forces on each coil is borne by a large surface area of the coil structure, where it is bonded to the radially outer mechanical support structure.
The term “solenoidal” is generally applied to describe cylindrical magnets made up of individual coils, although such cylindrical magnets may not be “solenoids” in the pure sense.